1. Field of Invention
This invention relates to a shell and U-tube vapor generating unit in which heat is supplied by a hot fluid flowing within the tubes, and more particularly, to an improved arrangement for the de-entrainment and blowdown of sediment.
2. Summary of the Prior Art
The use of shell and U-tube type vapor generating units, particularly in nuclear power plant steam generator applications, is well known.
Typically, the unit is contained within a vertically elongated pressure vessel of circular cross section and consists of a fluid heated inverted U-tube bundle disposed in the lower portion of the vessel within a circular shroud which forms an interior vapor generating chamber and an annular shaped downcomer bounded by the shroud and the vertical wall of the pressure vessel. Both ends of each tube are connected to a tubesheet located at one end of the vessel transversely of the longitudinal centerline of the vessel.
During operation of the unit a hot "primary" fluid, such as water, liquid metal or gas, enters one end of the tubes and flows through the tube bundle wherein it transfers heat before discharging at the opposite end of each tube. The leg of each U-tube receiving the hot fluid is generally characterized as a "hot leg". Conversely, the leg from which the cooled fluid is discharged is known as the "cold leg". The hot legs are usually grouped on one side of a vertical center plane of the vessel and the cold legs on the opposite side.
Water or another vaporizable "secondary" liquid descends through the annular downcomer to the tubesheet and radially enters the vapor generating chamber flowing into the spacing outside of the closely packed bundle of tubes, generally through a continuous circumferential opening between the bottom of the shroud and the upper surface of the tubesheet. The secondary liquid passes up over the outside of the tubes due to the thermal siphonic effect of the heat being transferred from the hot primary fluid, and is vaporized.
It is known that matter entrained in a flowing liquid separates therefrom and settles at points where the liquid velocity and the resulting turbulence are low. Tube failures and localized tube corrosion have been observed to be coincident with such sediment deposition. The velocity distribution of the secondary fluid, therefore, is of considerable importance in providing for sediment removal.
Blowdown pipes have been commonly utilized in vapor generating units to remove sediment deposited in the vicinity of the tubesheet by providing blowdown procedures in which the affected area receives a periodic or continuous flushing. Blowdown means have been located in the gap between the hot and cold legs. This location is desirable because it does not necessitate the omission of any tubes from a symmetrical tube bundle pattern which would result in the loss of heat transfer surface for a given unit volume. Experience has shown, however, that central location of the blowdown pipes has not effectively removed sediment that deposits on the tubesheet in the hot leg area. Recent analyses, moreover, indicate that the secondary fluid velocity distribution across the tubesheet from the periphery of the tube bundle is influenced by the higher vapor generation rate that occurs in the hot leg region of a U-tube bundle. It has been discovered that the effects of higher net vapor generation in the hot leg region is such that the area of null transverse velocity and low turbulence occurs in the midst of the hot leg region. Hence, sediment deposition in the hot leg region at the tubesheet appears to be predominant in these units. On more recent vapor generating units, therefore, blowdown arrangements have been provided in the hot leg region at the area of minimum velocity, necessitating the omission of a significant number of tubes from the tube bundle.
Thus, there exists a need to provide an efficient means of sediment removal at the tubesheet of a vapor generating unit of the type described above without the omission of tubes.